The Effects of Particle Reinforcements on Chip Formation and Machining Induced Damage in cutting of modified epoxy Carbon Fibre Reinforced Polymers

Multifunctional and multiphase materials, such as traditional carbon fibre reinforced polymers (CFRPs), are designed to achieve greater functionality compared to their individual constituent materials. Coupling individual material phases creates new hybrid materials with improved performance by reducing dimensions, weight, expense and energy consumption, while enhancing safety, design, and manufacturing versatility. The unique characteristics of particulate reinforcements such as size, shape, concentration and mechanical properties necessary to create a beneficial change to the polymer matrix (i.e., by increasing the fracture toughness of a brittle polymer), together with the advancement in polymer characterisation and simulation techniques, have generated a great deal of interest in the field of CFRPs with modified epoxy matrices. In manufacturing of epoxy-based composite materials, machining operations are often required to achieve tight geometric tolerances or ensure edge-of-part mechanical performance. Even though cutting process parameters are controlled to minimise machining induced damage, an in-depth assessment of the relation between material removal mechanism, material properties and machining induced damage is required. The aim of this project is to alter the mechanical properties of the epoxy resin through the introduction of particle reinforcement, to better characterise the subsequent chip formation process and machining induced damage in cutting of epoxy modified carbon fibre reinforced polymers. This is completed through a series of experimental studies which will facilitate the in-situ observations of material removal mechanism in cutting of silica and rubber modified epoxy CFRPs. A manufacturing technique is developed to include silica nanoparticles and rubber microparticles in the epoxy matrix of CFRPs. Tensile and fracture toughness tests were conducted to quantify the effect of particle reinforcements and results are further used in the discussion of the machining results. A novel orthogonal cutting rig coupled with a 2D High Speed Digital Image Correlation (DIC) system was developed to study the material removal mechanism of polymer and CFRP samples at a micro scale level. Current state-of-the-art 3D areal metrics are used to quantify machining induced damage, while microscopy is used to provide qualitative data of the machined surface. The extent of subsurface damage is assessed using micro – Computed Tomography (CT) scanning. Finally, to address the industrial application of this research, the machining performance of epoxy modified CFRPs under edge trimming conditions is analysed. It has been found that the addition of rubber microparticles and silica nanoparticles affects the material removal mechanism in both orthogonal cutting and edge trimming conditions as such the cutting forces and machining induced damage tend to decrease with the addition of rubber particles. Design of experiment (DoE) and analysis of variance (ANOVA) methods employed in the edge trimming study showed a statistical correlation between particle concentrations, machining variables, cutting forces and surface metrics. Experimental evidence gathered by white-light and Scanning Electron Microscopy (SEM) showed that rubber toughening mechanism ensured an efficient energy dissipation mechanism limiting crack propagation and extent of subsurface damage. The brittle state of silica and unmodified epoxy proved ineffective in reducing machining induced damage. This thesis provides fundamental work in the addition of particle reinforcements in the epoxy matrix of a CFRP material, with an in-depth assessment of the optimum cutting parameters and material properties needed to ensure low damage machining processes. Gaining understanding of the physics of chip formation process of particle modified epoxy composites will enable designers and engineers a greater ability to introduce particle reinforcements during Design for Manufacture (DFM) stages of product design leading towards damage-free machining of high-value composite parts.